This invention relates to novel aldehydic, cationic derivatives of natural occurring galactose containing polysaccharides obtained by site-specific oxidizing the selected polysaccharide with the enzyme galactose oxidase. This cation derivatized, enzyme oxidized product has an aldehyde function at a specific position of the polysaccharide and provides surprising strength properties when used as a paper additive.
The term "paper" as used herein includes sheet-like masses and molded products made from cellulosic materials which may be derived from natural sources as well as from synthetics such as polyamides, polyesters and polyacrylic resins and from mineral fibers such as asbestos and glass. In addition, papers made from combinations of cellulosic and synthetic materials are applicable herein. Paperboard is also included within the broad term "paper"
The modification of starch and other polysaccharides by many different methods to produce various cation and aldehyde containing polysaccharides as well as cationic-aldehyde containing derivatives is well known. Many of these modified polysaccharides have been used as paper additives to improve properties such as strength, drainage and pigment retention. However, it has been found that the modified polysaccharides are often not useful because they prematurely ies.
The cationic polysaccharides can be produced by reaction with reagents which will introduce a cationic group containing nitrogen, sulfur or phosphorus therein as disclosed in "Cationic Starches" by D. B, Solarek, in Modified Starches: Properties and Use, Chapter 8, 1986. Particularly useful cationic derivatives are the tertiary aminoalkyl starch ethers and the quaternary ammonium starch ethers.
Oxidative and non-oxidative methods have been used to introduce aldehyde groups into polysaccharides such as starches, gums and celluloses. The oxidative methods used have included treatment with periodic acid, periodates, or alkali metal ferrates. See U.S. Pat. No. 3,086,969 issued Apr. 23, 1963 to J. E. Slager which discloses an improved process for the preparation of a dialdehyde polysaccharide using periodic acid; U.S. Pat. No. 3,236,832 issued Feb. 22, 1966 to J. W. Opie et al. which discloses a method of preparing periodate modified polygalactomannan gums using periodic acid or the alkali metal salts thereof; U.S. Pat. No. 3,062,652 issued Nov. 6, 1962 to R. A. Jeffreys et al. which discloses the preparation of dialdehyde gums using periodate or periodic acid; and U.S. Pat. No. 3,632,082 issued Jan. 4, 1972 to J. N. BeMiller et. al. which discloses a method for oxidizing a carbohydrate (e.g., starch or cellulose) with an alkali metal ferrate.
The disadvantages of the oxidative method include degradation to lower molecular weight products and the formation of carboxyl groups due to further oxidation of the aldehyde groups. U.S. Pat. No. 3,553,193 issued Jan. 5, 1973 to D. H. LeRoy et al. describes a method for oxidizing starch using an alkali metal bromite or hypobromite under carefully controlled conditions. The resulting aldehyde is reported to have a substantially greater proportion of carbonyl groups, i.e., aldehyde groups, than carboxyl groups. It also discloses a method for selectively oxidizing the side chains of starch derivatives, e.g., an alkoxylated starch such as dihydroxypropyl starches, under the same process conditions whereby the underivatized starch hydroxy groups on the rings are substantially non-oxidized.
The presence of carboxylic groups in aldehyde starches has several disadvantages in addition to the obvious reduction in the degree of aldehyde substitution. This includes the introduction of hydrophilic properties due to the carboxyl groups, an upset in the cationic/anionic ratio when a cationic starch base is used (as in most papermaking wet end uses) and the possible formation of salts (in papermaking wet end use) which could give rise to ionic crosslinking.
The non-oxidative methods typically involve the chemical modification of the polysaccharide with an aldehyde-containing reagent. Generally, chemical modification is random or not site-specific. See U.S. Pat. No. 3,519,618 issued Jul. 7, 1970 to S. M. Parmerter and U.S. Pat. No. 3,740,391 issued Jun. 19, 1973 to L. L. Williams et al. which cover starch derivatives and U.S. Pat. No. 2,803,558 issued Aug. 20, 1957 to G. D. Fronmuller which covers a gum derivative. The starch derivative of Parmerter is prepared by reaction with an unsaturated aldehyde (e.g., acrolein) and has the structure: EQU Starch--O--CH(R.sup.1)--CH(R.sup.2)--CHO
where R.sup.1 and R.sup.2 are hydrogen, lower alkyls or halogen. The starch derivative of Williams is prepared by reaction with acrylamide followed by reaction with glyoxal and has the structure: ##STR1## The gum derivative of Fronmuller is prepared by treating the dry gum (e.g., locust bean or guar gum) with peracetic acid to reduce the viscosity, neutralizing and then reacting with glyoxal. Water soluble cellulose ethers (e.g., hydroxyethyl cellulose) have also been reacted with glyoxal or urea formaldehyde to give aldehyde-containing derivatives.
One of the disadvantages of introducing the aldehyde groups directly using an aldehyde-containing reagent is the possibility of the derivative crosslinking prior to use. This is a particular disadvantage when the products are being used to impart temporary wet strength to paper via a crosslinking reaction with the cellulose fibers. The Williams '391 patent, cited above, alludes to this problem when it notes that solutions of the glyoxalated polymers are stable for at least a week when diluted to 10% solids by weight and adjusted to pH of 3 (Col. 3, lines 60-63). The Parmerter patent notes that the starch aldehyde is a substantially non-crosslinked granular starch derivative and discusses the importance of the non-crosslinked character (col. 2, lines 40-45)
U.S. Pat. No. 4,675,394 issued Jun. 23, 1987 to D. B. Solarek et. al. discloses a non-oxidative method for introducing block aldehyde groups into starch. This method avoids the premature reaction of the aldehyde by introducing acetal groups which are easily hydrolyzed at low pH to form the aldehyde. However, the generation of the aldehydic function at low pH also degrades the polysaccharide backbone. For certain applications, this degradation is undesirable. Cationic aldehyde containing derivatives are further disclosed as being useful as paper strength additives.
U.S. Pat. No. 3,297,604 issued Jan. 10, 1967 to F. J. Germino discloses galactose containing polysaccharides which are oxidized chemically or enzymatically with galactose oxidase. This patent further discloses the use of these oxidized products as various intermediates for crosslinking polymers, both natural and synthetic, and for improving strength properties of cellulose and paper.
U.S. Pat. No. 4,031,307 issued Jun. 21, 1977 to R. N. De Martino et. al. discloses cationic polygalactomannan compositions and more particularly a process or producing quaternary ammonium ethers of polygalactomannan gums.
The composition of a cationic modified and site-specific oxidized natural polysaccharide has not been disclosed previously and furthermore it has now been found that surprising and unexpectedly improved paper strength properties are achieved when using such polysaccharides as strength additives.